Implemented bottom footnotes with backreferences (getzola#2480)

* Implemented bottom footnotes with backreferences

Fixes getzola#1285

* Added bottom_footnotes option to configuration.md

* Renamed fix_github_style_footnotes()

* Added tests for convert_footnotes_to_github_style()

* Changed test to plain html instead of Vec<Event>

* Added integration test for footnotes

* Applied suggested changes

Author: totikom

components/config/src/config/markup.rs

@@ -43,6 +43,8 @@ pub struct Markdown {
     pub external_links_no_referrer: bool,
     /// Whether smart punctuation is enabled (changing quotes, dashes, dots etc in their typographic form)
     pub smart_punctuation: bool,
+    /// Whether footnotes are rendered at the bottom in the style of GitHub.
+    pub bottom_footnotes: bool,
     /// A list of directories to search for additional `.sublime-syntax` and `.tmTheme` files in.
     pub extra_syntaxes_and_themes: Vec<String>,
     /// The compiled extra syntaxes into a syntax set
@@ -203,6 +205,7 @@ impl Default for Markdown {
             external_links_no_follow: false,
             external_links_no_referrer: false,
             smart_punctuation: false,
+            bottom_footnotes: false,
             extra_syntaxes_and_themes: vec![],
             extra_syntax_set: None,
             extra_theme_set: Arc::new(None),

components/markdown/src/markdown.rs

@@ -1,5 +1,7 @@
+use std::collections::HashMap;
 use std::fmt::Write;
 
+use crate::markdown::cmark::CowStr;
 use errors::bail;
 use libs::gh_emoji::Replacer as EmojiReplacer;
 use libs::once_cell::sync::Lazy;
@@ -239,6 +241,158 @@ fn get_heading_refs(events: &[Event]) -> Vec<HeadingRef> {
     heading_refs
 }
 
+fn convert_footnotes_to_github_style(old_events: &mut Vec<Event>) {
+    let events = std::mem::take(old_events);
+    // step 1: We need to extract footnotes from the event stream and tweak footnote references
+
+    // footnotes bodies are stored in a stack of vectors, because it is possible to have footnotes
+    // inside footnotes
+    let mut footnote_bodies_stack = Vec::new();
+    let mut footnotes = Vec::new();
+    // this will allow to create a multiple back references
+    let mut footnote_numbers = HashMap::new();
+    let filtered_events = events.into_iter().filter_map(|event| {
+        match event {
+            // New footnote definition is pushed to the stack
+            Event::Start(Tag::FootnoteDefinition(_)) => {
+                footnote_bodies_stack.push(vec![event]);
+                None
+            }
+            // The topmost footnote definition is popped from the stack
+            Event::End(TagEnd::FootnoteDefinition) => {
+                // unwrap will never fail, because Tag::FootnoteDefinition always comes before
+                // TagEnd::FootnoteDefinition
+                let mut footnote_body = footnote_bodies_stack.pop().unwrap();
+                footnote_body.push(event);
+                footnotes.push(footnote_body);
+                None
+            }
+            Event::FootnoteReference(name) => {
+                // n will be a unique index of the footnote
+                let n = footnote_numbers.len() + 1;
+                // nr is a number of references to this footnote
+                let (n, nr) = footnote_numbers.entry(name.clone()).or_insert((n, 0usize));
+                *nr += 1;
+                let reference = Event::Html(format!(r##"<sup class="footnote-reference" id="fr-{name}-{nr}"><a href="#fn-{name}">[{n}]</a></sup>"##).into());
+
+                if footnote_bodies_stack.is_empty() {
+                    // we are in the main text, just output the reference
+                    Some(reference)
+                } else {
+                    // we are inside other footnote, we have to push that reference into that
+                    // footnote
+                    footnote_bodies_stack.last_mut().unwrap().push(reference);
+                    None
+                }
+            }
+            _ if !footnote_bodies_stack.is_empty() => {
+                footnote_bodies_stack.last_mut().unwrap().push(event);
+                None
+            }
+            _ => Some(event),
+        }
+    }
+    );
+
+    old_events.extend(filtered_events);
+
+    if footnotes.is_empty() {
+        return;
+    }
+
+    old_events.push(Event::Html("<hr><ol class=\"footnotes-list\">\n".into()));
+
+    // Step 2: retain only footnotes which was actually referenced
+    footnotes.retain(|f| match f.first() {
+        Some(Event::Start(Tag::FootnoteDefinition(name))) => {
+            footnote_numbers.get(name).unwrap_or(&(0, 0)).1 != 0
+        }
+        _ => false,
+    });
+
+    // Step 3: Sort footnotes in the order of their appearance
+    footnotes.sort_by_cached_key(|f| match f.first() {
+        Some(Event::Start(Tag::FootnoteDefinition(name))) => {
+            footnote_numbers.get(name).unwrap_or(&(0, 0)).0
+        }
+        _ => unreachable!(),
+    });
+
+    // Step 4: Add backreferences to footnotes
+    let footnotes = footnotes.into_iter().flat_map(|fl| {
+        // To write backrefs, the name needs kept until the end of the footnote definition.
+        let mut name = CowStr::from("");
+        // Backrefs are included in the final paragraph of the footnote, if it's normal text.
+        // For example, this DOM can be produced:
+        //
+        // Markdown:
+        //
+        //     five [^feet].
+        //
+        //     [^feet]:
+        //         A foot is defined, in this case, as 0.3048 m.
+        //
+        //         Historically, the foot has not been defined this way, corresponding to many
+        //         subtly different units depending on the location.
+        //
+        // HTML:
+        //
+        //     <p>five <sup class="footnote-reference" id="fr-feet-1"><a href="#fn-feet">[1]</a></sup>.</p>
+        //
+        //     <ol class="footnotes-list">
+        //     <li id="fn-feet">
+        //     <p>A foot is defined, in this case, as 0.3048 m.</p>
+        //     <p>Historically, the foot has not been defined this way, corresponding to many
+        //     subtly different units depending on the location. <a href="#fr-feet-1">↩</a></p>
+        //     </li>
+        //     </ol>
+        //
+        // This is mostly a visual hack, so that footnotes use less vertical space.
+        //
+        // If there is no final paragraph, such as a tabular, list, or image footnote, it gets
+        // pushed after the last tag instead.
+        let mut has_written_backrefs = false;
+        let fl_len = fl.len();
+        let footnote_numbers = &footnote_numbers;
+        fl.into_iter().enumerate().map(move |(i, f)| match f {
+            Event::Start(Tag::FootnoteDefinition(current_name)) => {
+                name = current_name;
+                has_written_backrefs = false;
+                Event::Html(format!(r##"<li id="fn-{name}">"##).into())
+            }
+            Event::End(TagEnd::FootnoteDefinition) | Event::End(TagEnd::Paragraph)
+                if !has_written_backrefs && i >= fl_len - 2 =>
+            {
+                let usage_count = footnote_numbers.get(&name).unwrap().1;
+                let mut end = String::with_capacity(
+                    name.len() + (r##" <a href="#fr--1">↩</a></li>"##.len() * usage_count),
+                );
+                for usage in 1..=usage_count {
+                    if usage == 1 {
+                        write!(&mut end, r##" <a href="#fr-{name}-{usage}">↩</a>"##).unwrap();
+                    } else {
+                        write!(&mut end, r##" <a href="#fr-{name}-{usage}">↩{usage}</a>"##)
+                            .unwrap();
+                    }
+                }
+                has_written_backrefs = true;
+                if f == Event::End(TagEnd::FootnoteDefinition) {
+                    end.push_str("</li>\n");
+                } else {
+                    end.push_str("</p>\n");
+                }
+                Event::Html(end.into())
+            }
+            Event::End(TagEnd::FootnoteDefinition) => Event::Html("</li>\n".into()),
+            Event::FootnoteReference(_) => unreachable!("converted to HTML earlier"),
+            f => f,
+        })
+    });
+
+    old_events.extend(footnotes);
+    old_events.push(Event::Html("</ol>\n".into()));
+}
+
 pub fn markdown_to_html(
     content: &str,
     context: &RenderContext,
@@ -623,6 +777,10 @@ pub fn markdown_to_html(
             insert_many(&mut events, anchors_to_insert);
         }
 
+        if context.config.markdown.bottom_footnotes {
+            convert_footnotes_to_github_style(&mut events);
+        }
+
         cmark::html::push_html(&mut html, events.into_iter());
     }
 
@@ -641,11 +799,11 @@ pub fn markdown_to_html(
 
 #[cfg(test)]
 mod tests {
+    use super::*;
     use config::Config;
+    use insta::assert_snapshot;
 
-    use super::*;
     #[test]
-
     fn insert_many_works() {
         let mut v = vec![1, 2, 3, 4, 5];
         insert_many(&mut v, vec![(0, 0), (2, -1), (5, 6)]);
@@ -714,4 +872,106 @@ mod tests {
             assert_eq!(body, &bottom_rendered);
         }
     }
+
+    #[test]
+    fn no_footnotes() {
+        let mut opts = Options::empty();
+        opts.insert(Options::ENABLE_TABLES);
+        opts.insert(Options::ENABLE_FOOTNOTES);
+        opts.insert(Options::ENABLE_STRIKETHROUGH);
+        opts.insert(Options::ENABLE_TASKLISTS);
+        opts.insert(Options::ENABLE_HEADING_ATTRIBUTES);
+
+        let content = "Some text *without* footnotes.\n\nOnly ~~fancy~~ formatting.";
+        let mut events: Vec<_> = Parser::new_ext(&content, opts).collect();
+        convert_footnotes_to_github_style(&mut events);
+        let mut html = String::new();
+        cmark::html::push_html(&mut html, events.into_iter());
+        assert_snapshot!(html);
+    }
+
+    #[test]
+    fn single_footnote() {
+        let mut opts = Options::empty();
+        opts.insert(Options::ENABLE_TABLES);
+        opts.insert(Options::ENABLE_FOOTNOTES);
+        opts.insert(Options::ENABLE_STRIKETHROUGH);
+        opts.insert(Options::ENABLE_TASKLISTS);
+        opts.insert(Options::ENABLE_HEADING_ATTRIBUTES);
+
+        let content = "This text has a footnote[^1]\n [^1]:But it is meaningless.";
+        let mut events: Vec<_> = Parser::new_ext(&content, opts).collect();
+        convert_footnotes_to_github_style(&mut events);
+        let mut html = String::new();
+        cmark::html::push_html(&mut html, events.into_iter());
+        assert_snapshot!(html);
+    }
+
+    #[test]
+    fn reordered_footnotes() {
+        let mut opts = Options::empty();
+        opts.insert(Options::ENABLE_TABLES);
+        opts.insert(Options::ENABLE_FOOTNOTES);
+        opts.insert(Options::ENABLE_STRIKETHROUGH);
+        opts.insert(Options::ENABLE_TASKLISTS);
+        opts.insert(Options::ENABLE_HEADING_ATTRIBUTES);
+
+        let content = "This text has two[^2] footnotes[^1]\n[^1]: not sorted.\n[^2]: But they are";
+        let mut events: Vec<_> = Parser::new_ext(&content, opts).collect();
+        convert_footnotes_to_github_style(&mut events);
+        let mut html = String::new();
+        cmark::html::push_html(&mut html, events.into_iter());
+        assert_snapshot!(html);
+    }
+
+    #[test]
+    fn def_before_use() {
+        let mut opts = Options::empty();
+        opts.insert(Options::ENABLE_TABLES);
+        opts.insert(Options::ENABLE_FOOTNOTES);
+        opts.insert(Options::ENABLE_STRIKETHROUGH);
+        opts.insert(Options::ENABLE_TASKLISTS);
+        opts.insert(Options::ENABLE_HEADING_ATTRIBUTES);
+
+        let content = "[^1]:It's before the reference.\n\n There is footnote definition?[^1]";
+        let mut events: Vec<_> = Parser::new_ext(&content, opts).collect();
+        convert_footnotes_to_github_style(&mut events);
+        let mut html = String::new();
+        cmark::html::push_html(&mut html, events.into_iter());
+        assert_snapshot!(html);
+    }
+
+    #[test]
+    fn multiple_refs() {
+        let mut opts = Options::empty();
+        opts.insert(Options::ENABLE_TABLES);
+        opts.insert(Options::ENABLE_FOOTNOTES);
+        opts.insert(Options::ENABLE_STRIKETHROUGH);
+        opts.insert(Options::ENABLE_TASKLISTS);
+        opts.insert(Options::ENABLE_HEADING_ATTRIBUTES);
+
+        let content = "This text has two[^1] identical footnotes[^1]\n[^1]: So one is present.\n[^2]: But another in not.";
+        let mut events: Vec<_> = Parser::new_ext(&content, opts).collect();
+        convert_footnotes_to_github_style(&mut events);
+        let mut html = String::new();
+        cmark::html::push_html(&mut html, events.into_iter());
+        assert_snapshot!(html);
+    }
+
+    #[test]
+    fn footnote_inside_footnote() {
+        let mut opts = Options::empty();
+        opts.insert(Options::ENABLE_TABLES);
+        opts.insert(Options::ENABLE_FOOTNOTES);
+        opts.insert(Options::ENABLE_STRIKETHROUGH);
+        opts.insert(Options::ENABLE_TASKLISTS);
+        opts.insert(Options::ENABLE_HEADING_ATTRIBUTES);
+
+        let content = "This text has a footnote[^1]\n[^1]: But the footnote has another footnote[^2].\n[^2]: That's it.";
+        let mut events: Vec<_> = Parser::new_ext(&content, opts).collect();
+        convert_footnotes_to_github_style(&mut events);
+        let mut html = String::new();
+        cmark::html::push_html(&mut html, events.into_iter());
+        assert_snapshot!(html);
+    }
 }

components/markdown/src/snapshots/markdown__markdown__tests__def_before_use.snap

@@ -0,0 +1,10 @@
+---
+source: components/markdown/src/markdown.rs
+expression: html
+---
+<p>There is footnote definition?<sup class="footnote-reference" id="fr-1-1"><a href="#fn-1">[1]</a></sup></p>
+<hr><ol class="footnotes-list">
+<li id="fn-1">
+<p>It's before the reference. <a href="#fr-1-1">↩</a></p>
+</li>
+</ol>

components/markdown/src/snapshots/markdown__markdown__tests__footnote_inside_footnote.snap

@@ -0,0 +1,13 @@
+---
+source: components/markdown/src/markdown.rs
+expression: html
+---
+<p>This text has a footnote<sup class="footnote-reference" id="fr-1-1"><a href="#fn-1">[1]</a></sup></p>
+<hr><ol class="footnotes-list">
+<li id="fn-1">
+<p>But the footnote has another footnote<sup class="footnote-reference" id="fr-2-1"><a href="#fn-2">[2]</a></sup>. <a href="#fr-1-1">↩</a></p>
+</li>
+<li id="fn-2">
+<p>That's it. <a href="#fr-2-1">↩</a></p>
+</li>
+</ol>

components/markdown/src/snapshots/markdown__markdown__tests__multiple_refs.snap

@@ -0,0 +1,10 @@
+---
+source: components/markdown/src/markdown.rs
+expression: html
+---
+<p>This text has two<sup class="footnote-reference" id="fr-1-1"><a href="#fn-1">[1]</a></sup> identical footnotes<sup class="footnote-reference" id="fr-1-2"><a href="#fn-1">[1]</a></sup></p>
+<hr><ol class="footnotes-list">
+<li id="fn-1">
+<p>So one is present. <a href="#fr-1-1">↩</a> <a href="#fr-1-2">↩2</a></p>
+</li>
+</ol>

components/markdown/src/snapshots/markdown__markdown__tests__no_footnotes.snap

@@ -0,0 +1,6 @@
+---
+source: components/markdown/src/markdown.rs
+expression: html
+---
+<p>Some text <em>without</em> footnotes.</p>
+<p>Only <del>fancy</del> formatting.</p>

components/markdown/src/snapshots/markdown__markdown__tests__reordered_footnotes.snap

@@ -0,0 +1,13 @@
+---
+source: components/markdown/src/markdown.rs
+expression: html
+---
+<p>This text has two<sup class="footnote-reference" id="fr-2-1"><a href="#fn-2">[1]</a></sup> footnotes<sup class="footnote-reference" id="fr-1-1"><a href="#fn-1">[2]</a></sup></p>
+<hr><ol class="footnotes-list">
+<li id="fn-2">
+<p>But they are <a href="#fr-2-1">↩</a></p>
+</li>
+<li id="fn-1">
+<p>not sorted. <a href="#fr-1-1">↩</a></p>
+</li>
+</ol>

components/markdown/src/snapshots/markdown__markdown__tests__single_footnote.snap

@@ -0,0 +1,10 @@
+---
+source: components/markdown/src/markdown.rs
+expression: html
+---
+<p>This text has a footnote<sup class="footnote-reference" id="fr-1-1"><a href="#fn-1">[1]</a></sup></p>
+<hr><ol class="footnotes-list">
+<li id="fn-1">
+<p>But it is meaningless. <a href="#fr-1-1">↩</a></p>
+</li>
+</ol>